In the paper-making process, chemical pulping yields black liquor as a by-product. Black liquor contains inorganic cooking chemicals along with lignin and other organic matter that separates from wood during pulping in a digester. The black liquor is burned in a boiler. The two main functions of the boiler are to recover the inorganic cooking chemicals used in the pulping process and to make use of chemical energy in the organic portion of the black liquor to generate steam for a paper mill. As used herein, the term boiler includes a top-supported boiler that burns a fuel that, in the process, fouls heat transfer surfaces.
An example of a boiler used to burn black liquor to generate steam is a Kraft boiler. A Kraft boiler includes banks of heat exchangers at various levels in the furnace for extracting heat by radiation and convection from the furnace gases to generate heated fluids such as steam. Typically, the furnace gases first interact with heat exchangers in a superheater bank to generate superheated steam. The furnace gases then interact with heat exchangers in a generating bank to generate working steam. The generating bank may also be referred to as a boiler bank. Finally, the furnace gases interact with heat exchangers in an economizer bank, which generates lower temperature heated fluids. The banks of heat exchangers are constructed of an array of platens that are constructed of tubes that function as heat exchanger surfaces for conducting and transferring heat. While operating, heat exchanger surfaces are continually fouled by ash generated in the furnace chamber from burning fuels such as black liquor. The amount of fuel that can be burned in a boiler is often limited by the rate and extent of fouling on the surfaces of the heat exchangers. The fouling, including ash deposited on the heat exchanger surfaces, reduces the heat absorbed from fuel combustion, resulting in reduced exit steam temperatures in the fouled heat exchanger banks and high gas temperatures entering the next heat exchanger bank in the boiler. For example, fouling in the superheater bank results in decreased steam temperatures exiting the heat exchanger and increased furnace gas temperature entering the generating bank. The heat exchanger surfaces in the generating bank tend to be relatively narrow compared to the spacing in the superheater and economizer banks, which increases the likelihood of fouling in the generating bank as compared to fouling in the superheater and economizer banks.
Fouling can require a boiler to be shutdown for cleaning when either the exit steam temperature is too low for use in downstream equipment or the temperature entering the downstream heat exchanger bank, such as the generating bank downstream from the superheater bank, exceeds the melting temperature of the deposits, resulting in gas side pluggage of the downstream bank. In addition, fouling can eventually cause plugging in the upstream bank as well, such as the superheater bank. In order to remove the plugging from the heat exchanger banks, the burning process in the boiler must be stopped. Kraft boilers are particularly prone to the problem of fouling in the generating bank with ash deposits that must be removed for efficient operation, however the other heat exchanger banks may also become fouled. Three conventional methods of removing ash deposits from the heat exchanger banks in boilers such as Kraft boilers include: 1) sootblowing, 2) chill-and-blow, and 3) waterwashing. This application addresses only the first of these methods, sootblowing.
Sootblowing is a process that includes blowing deposited ashes off a heat exchanger surface that is fouled with ash deposits using blasts of steam from nozzles of a lance of a sootblower. Sootblowing is performed essentially continuously during normal boiler operation, with sootblowers in various locations in operation at different times. Sootblowing is usually carried out using steam. The steam consumption of an individual sootblower is typically 2-3 kg/s, and as many as four sootblowers may be operated simultaneously. Typical sootblower usage is about 3-7% of the steam production of the entire boiler. Thus, the sootblowing procedure consumes a large amount of thermal energy produced by the boilers being cleaned.
A typical sootblowing process utilizes a procedure known as sequence sootblowing, wherein sootblowers operate at predetermined intervals and in a predetermined order. The sootblowing procedure runs at this pace irrespective of the amount of fouling that may occur at any particular location in the heat exchanger. Often, this leads to plugging in areas of the heat exchanger that are insufficiently cleaned by the predetermined sootblowing sequence that cannot necessarily be prevented even if the sootblowing procedure consumes a high amount of steam. Each sootblowing operation reduces a portion of nearby ash deposits, but ash deposits that are not completely removed may nevertheless continue to build up over time. As ash deposits grow, sootblowing becomes gradually less effective and impairs heat transfer. When an ash deposit reaches a certain threshold where boiler efficiency is significantly reduced or combustion gases cannot be removed from the furnace, deposits may need to be removed by another cleaning process requiring the boiler to be shut down.